A thermal printer relies in using the Joule effect of a resistive point either to vary the color of a heat sensitive medium situated next to said point, or else to cause a dye to be transferred from one medium to another medium by melting or the like.
Technical mastery over materials such as ink and heat sensitive media makes it possible, by adjusting the amount of heat energy given off by the printing point, to obtain shades in the intensity of the colors transferred or in the gray levels obtained.
Unfortunately, on manufacture, the resistance of the heating points is not uniform from one point to another along a line, and it also suffers from drift over time. This uncertainty is of the order of plus or minus 25% about a mean or nominal value, and is a factor that makes it impossible to obtain the desired degree of accuracy in the amount of energy transferred to the sensitive medium for obtaining the desired shade.
Such a thermal print head comprises a line of electrically resistive heating points that may be constituted by a line of discrete points or by a line of "inter-digitated" points, i.e. a line in which there is no physical separation between heating points, the points being constituted by lengths of a continuous resistive line, which points are defined by conductors connected to ground, and have respective conductors connected to the middles thereof for connection to a power supply.
An unsatisfied need therefore exists for a thermal printer capable of producing shaded printing in color or in gray scale. An object of the present invention is to provide a thermal printer of the type outlined above and including a circuit for correcting for the resistance of each point in a line of heating points in order to guarantee printing accuracy with respect to the desired shade, while not degrading operating speed.